Nitrous oxide (N₂O) is a greenhouse gas with a long lifetime and is a chemical that depletes the stratospheric ozone. N₂O is mainly divided into natural sources (volume fraction of about 60%) and anthropogenic sources (volume fraction of about 40%). Anthropogenic emissions come mainly from the use of agricultural nitrogen fertilizers, animal manure emissions, fossil fuel combustion, and industrial processes. N₂O has been present in the atmosphere for more than 100 years, and the current imbalance between its generation and disappearance will lead to a further increase in N₂O concentrations. Although the N₂O concentration in the atmosphere is at a low level, its potential to make the atmosphere warmer is about 300 times more than that of the same amount of CO₂. Carbon monoxide also (CO) plays an important role in atmospheric chemistry, and its reaction with OH radicals can directly or indirectly affect the fate of some key greenhouse gases, such as methane (CH₄) and ozone (O₃). The amount of Earth infrared radiation absorbed by CO is limited, but the cumulative indirect radiation intensity cannot be ignored. Therefore, the real-time online monitoring of these two gases plays an important role in the analysis of their sources and their impact on global warming.

The rapid development of tunable diode laser absorption spectroscopy has made it become a commonly used method for gas monitoring, due to its advantages of high resolution, real-time in-situ measurement, fast response, and high sensitivity. This method has been widely used in the fields of environmental monitoring, industrial process analysis, and biological activity analysis. Hollow waveguides (HWGs) are a novel form of optical fiber with a hollow inner core that can be used as an absorption cell for gas measurements, and they are widely used in laser absorption spectroscopy technology. HWG is flexible and can be bent and folded. Moreover, it has the advantages of small size, light weight, high stability, and fast response speed compared with traditional gas absorber cells. Based on HWGs with a length of 5 m and using a quantum cascade laser at a center wavelength of 4.56 µm combined with the wavelength modulation technique, a two-gas sensing system based on mid-infrared laser absorption spectroscopy has been developed for the simultaneous measurement of N₂O and CO.

First, by analyzing the effects of different coupling methods on the second harmonic background noise and signal-to-noise ratio, the optimal lens-focused coupling is chosen, and the basic principles of coupling a Gaussian beam into a hollow waveguide are discussed. Second, the concentration linear response of the system is experimentally tested , and the different concentration values are linearly fitted with the mean peak of their corresponding second harmonic signals. The linearity (R²) is over 99.9%, which shows there is a strong linear relationship between the two parameters. Third, analyzing the stability of the system, the Allan deviation indicates that the detection limits of N₂O and CO can achieve 1.8×10^-9 and 1.3874×10^-10 at integration time of 74 s and 75 s, respectively. This system satisfies the monitoring requirements for N₂O and CO, and it has important applications for evaluating concentration changes of these gases in the atmosphere.

In this study, a gas sensing system for N₂O and CO via mid-infrared laser absorption spectroscopy is built based on a 5 m long HWG and using a quantum cascade laser with a center wavelength of 4.56 µm. The performance of the system is analyzed through a series of experiments, and the results show that the system has high sensitivity, which satisfies the requirements for the detection of the atmospheric greenhouse gases N₂O and CO. Further, its simple structure is conducive to further integration and optimization. The use of a cage structure to simplify the system and the installation of a temperature control box for the system to cope with the effects of large changes in ambient temperature can be considered in the future.